kernel/time/timecompare.c

   1 /*
   2  * Copyright (C) 2009 Intel Corporation.
   3  * Author: Patrick Ohly <patrick.ohly@intel.com>
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License
  16  * along with this program; if not, write to the Free Software
  17  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  18  */
  19
  20 #include <linux/timecompare.h>
  21 #include <linux/module.h>
  22 #include <linux/slab.h>
  23 #include <linux/math64.h>
  24
  25 /*
  26  * fixed point arithmetic scale factor for skew
  27  *
  28  * Usually one would measure skew in ppb (parts per billion, 1e9), but
  29  * using a factor of 2 simplifies the math.
  30  */
  31 #define TIMECOMPARE_SKEW_RESOLUTION (((s64)1)<<30)
  32
  33 ktime_t timecompare_transform(struct timecompare *sync,
  34                               u64 source_tstamp)
  35 {
  36         u64 nsec;
  37
  38         nsec = source_tstamp + sync->offset;
  39         nsec += (s64)(source_tstamp - sync->last_update) * sync->skew /
  40                 TIMECOMPARE_SKEW_RESOLUTION;
  41
  42         return ns_to_ktime(nsec);
  43 }
  44 EXPORT_SYMBOL_GPL(timecompare_transform);
  45
  46 int timecompare_offset(struct timecompare *sync,
  47                        s64 *offset,
  48                        u64 *source_tstamp)
  49 {
  50         u64 start_source = 0, end_source = 0;
  51         struct {
  52                 s64 offset;
  53                 s64 duration_target;
  54         } buffer[10], sample, *samples;
  55         int counter = 0, i;
  56         int used;
  57         int index;
  58         int num_samples = sync->num_samples;
  59
  60         if (num_samples > sizeof(buffer)/sizeof(buffer[0])) {
  61                 samples = kmalloc(sizeof(*samples) * num_samples, GFP_ATOMIC);
  62                 if (!samples) {
  63                         samples = buffer;
  64                         num_samples = sizeof(buffer)/sizeof(buffer[0]);
  65                 }
  66         } else {
  67                 samples = buffer;
  68         }
  69
  70         /* run until we have enough valid samples, but do not try forever */
  71         i = 0;
  72         counter = 0;
  73         while (1) {
  74                 u64 ts;
  75                 ktime_t start, end;
  76
  77                 start = sync->target();
  78                 ts = timecounter_read(sync->source);
  79                 end = sync->target();
  80
  81                 if (!i)
  82                         start_source = ts;
  83
  84                 /* ignore negative durations */
  85                 sample.duration_target = ktime_to_ns(ktime_sub(end, start));
  86                 if (sample.duration_target >= 0) {
  87                         /*
  88                          * assume symetric delay to and from source:
  89                          * average target time corresponds to measured
  90                          * source time
  91                          */
  92                         sample.offset =
  93                                 (ktime_to_ns(end) + ktime_to_ns(start)) / 2 -
  94                                 ts;
  95
  96                         /* simple insertion sort based on duration */
  97                         index = counter - 1;
  98                         while (index >= 0) {
  99                                 if (samples[index].duration_target <
 100                                     sample.duration_target)
 101                                         break;
 102                                 samples[index + 1] = samples[index];
 103                                 index--;
 104                         }
 105                         samples[index + 1] = sample;
 106                         counter++;
 107                 }
 108
 109                 i++;
 110                 if (counter >= num_samples || i >= 100000) {
 111                         end_source = ts;
 112                         break;
 113                 }
 114         }
 115
 116         *source_tstamp = (end_source + start_source) / 2;
 117
 118         /* remove outliers by only using 75% of the samples */
 119         used = counter * 3 / 4;
 120         if (!used)
 121                 used = counter;
 122         if (used) {
 123                 /* calculate average */
 124                 s64 off = 0;
 125                 for (index = 0; index < used; index++)
 126                         off += samples[index].offset;
 127                 *offset = div_s64(off, used);
 128         }
 129
 130         if (samples && samples != buffer)
 131                 kfree(samples);
 132
 133         return used;
 134 }
 135 EXPORT_SYMBOL_GPL(timecompare_offset);
 136
 137 void __timecompare_update(struct timecompare *sync,
 138                           u64 source_tstamp)
 139 {
 140         s64 offset;
 141         u64 average_time;
 142
 143         if (!timecompare_offset(sync, &offset, &average_time))
 144                 return;
 145
 146         if (!sync->last_update) {
 147                 sync->last_update = average_time;
 148                 sync->offset = offset;
 149                 sync->skew = 0;
 150         } else {
 151                 s64 delta_nsec = average_time - sync->last_update;
 152
 153                 /* avoid division by negative or small deltas */
 154                 if (delta_nsec >= 10000) {
 155                         s64 delta_offset_nsec = offset - sync->offset;
 156                         s64 skew; /* delta_offset_nsec *
 157                                      TIMECOMPARE_SKEW_RESOLUTION /
 158                                      delta_nsec */
 159                         u64 divisor;
 160
 161                         /* div_s64() is limited to 32 bit divisor */
 162                         skew = delta_offset_nsec * TIMECOMPARE_SKEW_RESOLUTION;
 163                         divisor = delta_nsec;
 164                         while (unlikely(divisor >= ((s64)1) << 32)) {
 165                                 /* divide both by 2; beware, right shift
 166                                    of negative value has undefined
 167                                    behavior and can only be used for
 168                                    the positive divisor */
 169                                 skew = div_s64(skew, 2);
 170                                 divisor >>= 1;
 171                         }
 172                         skew = div_s64(skew, divisor);
 173
 174                         /*
 175                          * Calculate new overall skew as 4/16 the
 176                          * old value and 12/16 the new one. This is
 177                          * a rather arbitrary tradeoff between
 178                          * only using the latest measurement (0/16 and
 179                          * 16/16) and even more weight on past measurements.
 180                          */
 181 #define TIMECOMPARE_NEW_SKEW_PER_16 12
 182                         sync->skew =
 183                                 div_s64((16 - TIMECOMPARE_NEW_SKEW_PER_16) *
 184                                         sync->skew +
 185                                         TIMECOMPARE_NEW_SKEW_PER_16 * skew,
 186                                         16);
 187                         sync->last_update = average_time;
 188                         sync->offset = offset;
 189                 }
 190         }
 191 }
 192 EXPORT_SYMBOL_GPL(__timecompare_update);